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Yakima you can drive the whole canyon, and its more intresteing in October when the Sheep start rutting. The Methow is very beautiful, but then again you can drive it all. Lots of homes along the river anyway, so the ambience is more for the valley. If I were to do a float, I think I would look into the Deschutes. You can have a big float through big country and camp a couple days on the river. September would be a very pleasant time to do it, though it might be hard to book an outfitter as the stealhead are going pretty strong that time of year. A very pleasant trip and there isn't a highway right next to you.